I have been searching for a suitable translation for this, but the best I have gotten is the word ghItlh which is a verb meaning "write, engrave, incise, mark (upon)." I am unsure if there is some prefix or suffix that can be affixed to the word to make it mean a marking, engraving, etc. Any suggestions are appreciated.
The word [ngutlh] is listed by the Klingon Language Institute as meaning "letter, written character, glyph". This word was introduced at the 2016 Saarbrucken QepHom, a meeting of the KLI for which Marc Orkrand, the creator of the Klingon language, provides new words.
For upper and lower case letters, ngutlh tIn (literally "big letter") and ngutlh mach ("small letter") can be used respectively.